1. Field of the Invention
The present invention relates to a signal transmission line for a millimeter-wave band, and more particularly, to a signal transmission line for a millimeter-wave band in a metal thin film form, which is capable of efficiently transferring an electrical signal of about 57 to 63 GHz generated from a monolithic microwave integrated circuit (MMIC) mounted on a dielectric substrate.
This work was supported by the IT R&D program of MIC (Ministry of Information and Communication)/IITA (Institute for Information Technology Advancement) [2005-S-039-02, SoP (System on Pakage) for 60 GHz Pico cell Communication] in Korea.
2. Discussion of Related Art
FIG. 1 is a perspective view illustrating a conventional signal transmission line for a millimeter-wave band, and FIG. 2 is a graph showing frequency-dependent reflection and transmission characteristics of FIG. 1.
Referring to FIG. 1, the conventional signal transmission line for a millimeter-wave band in a metal thin film form includes a transmission line 4 formed on a dielectric substrate 2. The transmission line 4 has a single metal surface and is in the form of a waveguide. The transmission line 4 is connected to a connection pad 3a of a monolithic microwave integrated circuit (MMIC) 3 mounted on the dielectric substrate 2, via one wire 1.
The signal transmission line has an excellent low-frequency characteristic. The signal transmission line, however, has a poor resistance characteristic at a frequency of about 10 GHz or more because of its parasitic capacitance and parasitic inductance components, resulting in a poor transmission characteristic as in FIG. 2. FIG. 2 shows the transmission characteristic that moves away from 0 dB and the reflection characteristic that moves toward 0 dB as the signal frequency increases. The ideal transmission characteristic of a transmission is 0 dB, i.e., the output signal is the same as the input signal. On the other hand, the reflection characteristic refers to a ratio of an input signal over a reflected signal and is considered poor if it approaches 0 dB.
To solve this problem associated with the signal transmission line in a simple metal thin film form, a Wilkinson power divider which has two transformer lines for signal transmission is used.
The Wilkinson power divider divides one input power into two output powers. The Wilkinson power divider includes a concentration element and a distribution element. Recent increases of radio communication frequencies require that the elements and accordingly the power divider are small.
FIG. 3 is a circuit diagram illustrating a structure of a typical Wilkinson power divider. Referring to FIG. 3, the Wilkinson power divider includes an input line 10 having an impedance value of 50Ω, transformer lines 20 branched into two transformer lines from the input line 10 and having an impedance value of 70.7Ω, and output lines 30 having an impedance value of 50Ω and respectively connected to ends of the transformer lines 20.
The Wilkinson power divider further includes an isolation resistor 40 of 100Ω connected between the output lines 30. This isolation resistor 40 serves to improve isolation between the output terminals. The respective lines 10, 20 and 30 are formed of a material having an excellent conductivity. Generally, the isolation resistor 40 is a chip resistor or a thin film resistor.
Parasitic components, such as parasitic capacitance and parasitic inductance, are inevitably created when the isolation resistor 40 is connected between the output lines 30. The parasitic components are created irrespective of a manufacturer's intention. Such parasitic components greatly degrade performance of the power divider as an operation frequency increases.